So the phrase 'Sweet Medival Bitcoin' is used in the blurb that inspired the plague fellow (or maybe kaktus wrote that based on the plague fellow, I don't know) and I've heard the phrase used on chat at least once. Being somewhat literally minded, I started wondering how medieval bitcoin would actually work, and here's a rough draft. Let me know what you think.

I don't know what a "low value locker" is, but if this SCP belongs to Darke should it not get a little more protection than your average anomalous item? Also, as it's many items are all of them kept in the same locker, on the same site?

kept along side SCP-35xx

alongside

SCP-35xx-A refers to a collection of 12 vellum codices

I think there should be a sentence saying that "SCP-35xx is a collection of 36 items" or something, rather than jumping straight into 35xx-A.

SCP-35xx-C are quills made from the flight feathers of Black Swans.

The other 2 get specific numbers, this doesn't. I think they need one for consistency.
Also, I don't think "Black Swans" should be capitalised

the transaction will automatically appear as crossed out

I don't get this, does it cross out when you've finished writing and removed quill from paper?

It appears that

I don't like "it appears", maybe "it's been theorised" or even just "SCP-35xx was used"

Darke Trading Co

"Darke trading Co."

giving them a significant advantage

I think this needs an "over other businesses"

Curiously, it also appears that,

Why is this "curious", but the rest of the effects aren't?
Again, I don't like 'appears'. They did do that and the ledgers prove it.

it also appears that, due to the unprecedented security provided by SCP-35xxx, that authors

two "that"s, only one is needed.

oddly similar to modern debit cards and cryptocurrencies

I don't think the Foundation would point out that it's odd, because everything about the SCP is odd.

One book keeper even remarks on the absurdity of burning real whale oil just so that they can see well enough to record an imaginary currency.

Personally, this is too on-the-nose. I'm sure they would say this, but I don't think it belongs in a professional document.

It appears to have originated in the early to mid 15th century

How do we know this? Is that when the first entry was dated?

It was also known to engage in privateering and occasionally outright piracy.

Do they write about their piracy in the ledgers? That seems like the kind of thing you'd keep out of the books, even if they were anomalous books.

often being the only reliable source for anomalous goods and services

Am I reading this addendum wrong? I thought this was stuff they found specifically in the books. Why would they write this down and why would the Foundation believe it?

one of their shoppes

As this bit isn't a quote should it be "shops"?

few of which make any sense without context

Do we know any of the contexts? If not, how do we know they made sense in context?

possibly due to a multitude of CK-class reality shifts that have likely occurred since the ledger was written.

This seems like kind of a leap in logic. "They must be contradictory because all of reality shifted"? Maybe they just made something up to cover up their shady dealings.

one day be of use to us.

The Foundation, rather than "us".
Also, the history lesson kind of is useful already, right?